What is BSP? and what is the size of a BSP thread fitting? Bsp thread sizes mm

British Standard Pipe (BSP) is an established technical standard for screw threads.

BSP has been adopted internationally for interconnecting and sealing pipes and fittings by mating an external (male) thread with an internal (female) thread.

It has been adapted as standard in plumbing and pipe fitting in Australia and the Commonwealth countries.

Identify the size of a BSP thread fitting

We generally refer to a fitting by size, a 1” BSP nipple is also known as a 25mm BSP nipple but this actually refers to the inside diameter. The outside diameter for a 1” BSP fitting is not 1” or 25mm it is in fact 33.249mm at the widest point.

To identify exactly which fitting you have, measure across the thread where the red arrows are on the image below to get the “Actual Major Diameter”.

Refer to the “Actual Major Diameter” column on the chart and the corresponding columns for the BSP fitting size and the Metric size.

For example if the fitting measured 33.249 mm, it is a 1” BSP fitting or a 25mm Metric Fitting.

BSP Sizing Table

Metric Name BSP Name

Actual Major (Diameter in mm)

 Actual Major (Diameter in inches)
6mm 1/8"

 9.728 mm

8mm 1/4"

 13.157 mm

10mm 3/8"

 16.662 mm

15mm 1/2" 20.995 mm 0.825
20mm 3/4" 26.441 mm 1.041
25mm 1" 33.249 mm 1.309
32mm 1 1/4" 41.91 mm 1.65
40mm 1 1/2" 47.803 mm 1.882
50mm 2" 59.614 mm 2.347
65mm 2 1/2" 75.184 mm 2.96
75mm or 80mm 3" 87.884 mm 3.46
100mm 4" 113.03 mm 4.45
125mm 5" 138.43 mm 5.45
150mm 6" 163.83 mm 6.45

On a side note, NPT stands for National Pipe Thread and is an American standard thread.

Happy threading! Welcome any questions & comments and if there are any other topics you would like to know more!